On the other front, pathogens almost always evolve quickly to produce milder forms of disease - a pandemic will tend toward less-and-less lethality. A pathogen strain that keeps a host up-and-about will spread much more efficiently than one that quickly disables the host. Pathogens wiping out whole populations would require a particular type of population: confined (domestic animals are vulnerable, especially on modern factory farms, and cities are bad places to be) or already vulnerable (distemper might wipe out African lions, who are already in serious trouble).